There has recently been much work in the field of antibody-based assays to detect and measure the presence of particular substances. These types of assays often have several advantages over other methods of performing similar analysis, including high sensitivity in detecting small concentrations of a substance, high specificity for particular substances to be detected, and wide application to a large number of substances. Such tests rely on the ability of an antibody to bind to a particular antigen or other molecule. The bound antigen/antibody pair is then detected. Several methods exist for performing such detection of antigen/antibody complexes, for example, tagging the antigen or antibody with a flourescent or radioactive tracer which can be detected.
One method of detecting such an antigen/antibody complex involves the use of a matrix which contains an antigen or antibody which is immobilized in the matrix. In this method either the antibody or antigen may be the bound material in the matrix. More generally, such analyses can be performed using any type of ligand/anti-ligand pair, and the discussion of the operation of exemplary embodiments in terms of antibody/antigen pairs or other examples should not be considered to be a limitation on the applicability of the techniques of the present invention. The antigen or antibody which is bound will be referred to as the receptor and the antigen or antibody which is being detected will be called the analyte. The matrix including the bound receptors will be called the bioregion.
To detect or measure the presence of an analyte, it is typically diluted in a conductive solution such as a saline solution. The solution containing the analyte is then passed through the bioregion. The conductivity of the bioregion is measured by passing a current through the bioregion while measuring the voltage drop across the bioregion. As the analyte molecules are bound to the receptors in the bioregion, the cross-sectional area, and hence the volume, through which the current passes is reduced, and the conductivity of the biroregion decreases proportionately. Methods, sensing apparatus, and bioregions for performing these types of measurements are described in detail in a patent application Ser. No. 691,271 of D. Mitchell and R. Mitchell entitled "Measurement of Ligand/Antiligand Interactions Using Bulk Conductivity" filed Jan. 14, 1985 and incorporated by reference herein. This method of detecting and measuring an analyte in solution has many advantages of previously known methods for detecting substances, as described in the aforesaid application. Measuring the change in conductivity of such a bioregion presents novel problems which the present invention is directed towards solving.